Long time ago but basically involved preparation of flight trials instructions (FTI) for my own experiments as well as those of colleagues. Liason with TP. Approval from 'boss' (always a military TP - always involved stuff like 'how fast? 160? I'll give you 120!). Fly as observer to direct trial. Operate data acquisition kit in flight. Work with TP in flight. Data reduction (kneepad and tape - MODAS in my day). Data processing post flight. Re-write FTI and repeat as required.
Sounds easy as I type it now but my first time with a certain TP in our Lynx was like my Instrument Rating training - miles behind!
A marvellous time in my life though, working with some of the most professional, considerate and genuinely nice people I've ever met. The only time I've every really worked in or as a team.
The FTE though was the aircraft manager. He did all of the above plus liase with maintenance people. Calibrate the experimental fit. Design and oversee kit fits. Manage airworthiness and flight safety. The list goes on.
I once had to hang out of the side of a Lynx at 10000' with a cricket ball on some string, trying to con the pilot into a still-air hover. So the job could go from quite esoteric considerations, to some really practical flying.
In the UK I still think you really need to get into QinetiQ at Boscombe. However, I never knew any FTE's or FTO's who were military - all civvys.
I had a ball and learned my airmanship as an FTO. I hope that you can find a path to doing the job. Good luck